Congestive heart failure, hypertension, diabetes, and chronic renal failure have many different initial causes; however, all follow a common pathway in their progression to end-stage diseases. The common pathway is renal sympathetic nerve hyperactivity. Renal sympathetic nerves serve as the signal input pathway to higher sympathetic centers located in the spinal cord and brain via afferent renal nerve activity, increasing systemic sympathetic tone; meanwhile, through efferent activity, renal nerves and arteries participate in sympathetic hyperactivity in response to signals from the brain, further increasing systemic sympathetic tone (Dibona and Kopp, 1977). Sympathetic activation can initially be beneficial but eventually becomes maladaptive. In a state of sympathetic hyperactivity, a number of pathological events take place: abnormalities of hormonal secretion such as increased catecholamine, renine and angiotensin II levels, increased blood pressure due to peripheral vascular constriction and/or water and sodium retention, renal failure due to impaired glomerular filtration and nephron loss, cardiac dysfunction and heart failure due to left ventricular hypertrophy and myocyte loss, stroke, and even diabetes. Therefore, modulation (reduction/removal) of this increased sympathetic activity can slow or prevent the progression of these diseases. Recently, renal nerve denervation using high radio frequencies has become a recognized method to treat drug resistant hypertension (Esler et al., 2010 and Krum et al., 2009) and glucose metabolism abnormality (Mahfoud, 2011). However, certain methodologies by which renal nerve ablation or denervations are performed are either primitive, or are conducted in a manner whereby the medical professional operates with undue uncertainty respecting the location of the renal nerves critical in the disease pathway. The present invention seeks to rectify certain of these problems.
Renal Sympathetic Nerve Hyperactivity and Hypertension
Renal sympathetic nerve hyperactivity's contribution to the development and perpetuation of hypertension has been systematically investigated. This connection has been explored due in large part to the fact that, despite the availability of various pharmaceutical products and combination pharmaceutical products, and resources to assist patients' lifestyle changes, the rate of treatment of hypertension has remained surprisingly low. In particular, approximately ⅓ of hypertensive patients are not fully responsive to even optimized drug therapy and the measured blood pressure range amongst this cohort remains abnormal. This manifestation is called drug resistant hypertension. In approximately half of hypertensive patients, blood pressure remains higher than accepted treatment target levels. Amongst these patents with “essential” hypertension (i.e. persistent and pathological high blood pressure for which no specific cause can be found), it has been suggested that underlying pathophysiologies which are non-responsive to current treatment regimens exist. Further, it has been noted in such patients that efferent sympathetic renal nerve outflow stimulates renin release, increases tubular sodium reabsorption, and reduces renal blood flow, while afferent nerve signals from the kidney modulate central sympathetic outflow and thereby contribute to regulation of sodium and water metabolism, vascular tone/resistance and blood pressure.
Various data have confirmed the positive effects of renal nerve blocking on decreasing hypertension; data have further confirmed the connection between increased sympathetic nervous system activity and hypertension. In particular, studies have shown renal dysfunction as a mechanism of increased sympathetic nervous system activity leading to hypertension (Campese, 2002; Ye, 2002), that blocking renal nerve activity controls hypertension in animals with chronic renal insufficiency (Campese, 1995), and that surgical renal denervation performed to eliminate intractable pain in patients with polycystic kidney disease also eliminates hypertension (Valente 2001). Additional studies have identified increased noradrenaline spillover into the renal vein as the culprit in essential hypertension (Esler et al., 1990), and have shown that denervation by nephrectomy eliminates hypertension in humans on dialysis with severe hypertension refractory to multi-drug therapy (Converse 1992). Renal denervation has also been shown to delay or prevent the development of many experimental forms of hypertension in animals (e.g. spontaneously hypertensive rats (SHR), stroke prone SHR, New Zealand SHR, borderline hypertensive rats (BHR), Goldblatt 1K, 1C (rat), Goldblatt 2K, 2C (rat), aortic coarctation (dogs), aortic nerve transection (rat), DOCA-NaCL (rat, pig), Angiotensin II (rat, rabbit), fat feeding—obesity (dog), renal wrap (rat)) (DiBona and Kopp, 1997).
Certain previous efforts at decreasing refractory hypertension focused on a therapeutic drug approach, and in particular, the local administration of nerve blocking agents, such as local anesthetics, ketamine, tricyclic antidepressants, or neurotoxins, at the site of the nerve(s).
Studies performed in canines demonstrated proof-of-concept with regard to such a therapeutic drug approach. In one study, a total of eleven (11) dogs that had micro-embolization performed to induce acute heart failure were utilized to gather data; eight (8) dogs were treated with a renal nerve block created by injecting 10 ml of bupivacaine (Marcaine®) inside the Gerota's fascia, while three (3) served as controls. Urine output, as measured every fifteen (15) minutes, significantly increased in the bupivacaine-treated animals as compared with controls, and both natriuresis and diuresis were observed, confirming the physiologic basis for an antihypertensive effect. The same results were found in six (6) other dogs with micro-embolization resulting in chronic heart failure (Vigilance 2005).
Renal Sympathetic Nerve Hyperactivity, Insulin Sensitivity and Glucose Metabolism
Renal nerve hyperactivity is also posited to play a role in insulin sensitivity and glucose metabolism. Specifically, an increase in noradrenaline release accompanying renal nerve hyperactivity results in reduced blood flow, which in turn is associated with reduced glucose uptake. This indicates an impaired ability of cells to transport glucose across their membranes. Renal nerve hyperactivity is related to a neurally mediated reduction in the number of open capillaries, so that there is an increased distance that insulin must travel to reach the cell membrane from the intravascular compartment. Insulin-mediated increases in muscle perfusion are reduced by approximately 30% in insulin-resistant states. Consequently there is a direct relationship between muscle sympathetic nerve activity and insulin resistance, and an inverse relationship between insulin resistance and the number of open capillaries. (Mahfoud, et al., 2011). Renal sympathetic nerve hyperactivity is thus associated with certain aspects of diabetes mellitus and/or metabolic syndrome; sympathetic hyperactivity induces insulin resistance and hyperinsulinemia, which in turn produces additional sympathetic activation. Studies have been performed evaluating the effects of renal denervation on diabetic criteria.
A study by Mahfoud et al. (2011) tested the effect of renal denervation on patients who had type 2 diabetes mellitus, as well as high blood pressure of ≧160 mm Hg (or ≧150 mm Hg for patients with type 2 diabetes mellitus) despite being treated with at least 3 anti-hypertensive drugs (including 1 diuretic). At baseline and at follow-up visits taking place at one (1) and three (3) months after the procedure, blood chemistry, and fasting glucose, insulin, C peptide, and HbA1c were measured, while an oral glucose tolerance test (OGTT) was performed at baseline and after 3 months. Plasma glucose concentration was assessed with the glucose-oxidase method, while plasma insulin and C-peptide concentrations were measured by a chemiluminescent assay. Three months after denervation, diabetic indicators had substantially improved. At baseline, 13 patients in the treatment group had insulin levels ≧20 μIU/mL. Treatment decreased this number by 77% (n=10), with no changes in the control group. Insulin sensitivity also increased significantly after renal denervation. In 34 patients (test group, n=25; control group, n=9), the OGTT at baseline revealed 8 patients with impaired fasting glycemia, 18 patients with impaired glucose tolerance, and 8 patients with diabetes mellitus. After the procedure, 7 of 25 patients showed improvement in OGTT. The number of patients diagnosed with diabetes mellitus on the basis of OGTT was reduced by 12% (n=3); and the number of patients with normal glucose tolerance increased by 16% (n=4). Patients in the control group had no significant changes in glucose or insulin metabolism during follow-up.
The Mahfoud et al. study thus conclusively demonstrated that the renal sympathetic nervous system is an important regulator of insulin resistance and shows that renal nerve ablation substantially improves insulin sensitivity and glucose metabolism.
Renal Nerve Ablation Test Studies
During 1950s, surgical sympathectomy was utilized in humans as a treatment for severe hypertension before the availability of antihypertensive medicine (Smithwick and Thompson, 1953). However, such surgical renal denervation was extremely invasive and involved a major surgical procedure; therefore, it had great limitations in clinical practice (DiBona, 2003).
Recently, endovascular catheter technologies have been preferably utilized to create selective denervation in the human kidney. The renal nerves primarily lay outside the vessel tunica media, within the renal artery adventitial space. Consequently, radiofrequency energy, laser energy, high intensive focused ultrasound and alcohol can be delivered to renal artery walls, and cryoablative techniques likewise utilized on renal artery walls, via the renal artery lumen, to ablate sympathetic renal nerves.
The first human study of renal nerve ablation by catheter methodologies took place on hypertensive patient test subjects in 2009. Patient test subjects were enrolled whose standing blood pressure (SBP) was more than or equal to 160 mmHg despite the patient being on more than three anti-hypertensive medications (including diuretics), or who had a confirmed intolerance to anti-hypertensive medications (Krum et al., 2009). In this study of forty-five (45) patients overall baseline patient blood pressure consisted of (mmHg) of 177/101±20/15. Among enrolled patients, 89% of patients responded to renal denervation therapy and observed a reduction in blood pressure.
In order to assess whether renal denervation was effectively performed, after renal nerve ablation, renal noradrenaline spillover was measured to determine the success of the sympathetic denervation. Blood pressure was measured at baseline, and at 1 month, 3 months, 6 months, 9 months, and 12 months after the procedure. At each time point, decreases in both systolic and diastolic pressure were registered, with decreases continuing with the passage of time. Post-procedure, an overall decrease in total body noradrenaline spillover of 28% (p=0.043) was shown amongst the 45 test subjects, of which approximately one third was attributable to the renal sympathetic denervation. Treatment was delivered without complication in 43/45 patients, with no chronic vascular complications.
Current Protocols in Renal Denervation
After the Krum et al. study, there have been established certain accepted methodologies for performing renal nerve ablation through catheter means, though said methodologies comprise some variation. Typically, renal nerve ablation comprises catheter-based methods in which a patient is administered four (4) to six (6) two-minute radio frequency (RF) treatments per renal artery, with the radio frequency being generated by a radio frequency (RF) generator, which is automated, low-power, and has built-in safety algorithms. The radio frequencies, usually of 5-8 watts, are administered by catheter in the renal artery through movement of the catheter distal to the aorta to proximal to the aorta with application of the radio frequencies in spaced increments of 5 mm or more.
In the aforementioned Mahfoud et al. diabetes study, the following specific ablation protocol was followed: a treatment catheter was introduced into each renal artery by use of a renal double curve or left internal mammary artery guiding catheter; radiofrequency ablations lasting up to 2 minutes each were applied with low power of 8 watts to obtain up to 6 ablations separated both longitudinally and rotationally within each renal artery. Treatments were delivered from the first distal main renal artery bifurcation to the ostium. Catheter tip impedance and temperature were constantly monitored, and radiofrequency energy delivery was regulated according to a predetermined algorithm.
Endovascular catheter procedures such as those enumerated above are intended to preserve blood flow and minimize endothelial injury, while focal ablations spaced along the renal vessel allow for rapid healing. The resultant nerve ablation simultaneously diminishes the renal contribution to systemic sympathetic activation and the efferent effects of sympathetic activation of the kidney while offering a clinically durable result.
Functionally, the optimized goal of ablation of the renal arteries is to selectively disable the renal sympathetic (both afferent and efferent) nerves without impairing sympathetic signaling to other organs, and to precisely deliver energies to the locations in which renal sympathetic nerves are distributed in order to denervate the nerves. At present, renal nerve ablation is done in a “blind” fashion—that is, before the ablation radiofrequency is delivered, the physician who performs the procedure does not know where the renal sympathetic nerves are distributed so that the whole length of renal artery is ablated; furthermore, whether renal nerves have really been ablated or not can only be confirmed by measuring a secondary effect—i.e. norepinephreine spillover, after completion of the procedure. At present, approximately 89% of patients respond to renal denervation treatment (Krum et al., 2009 and Esler et al. 2010). However, these data were determined by measurements of patient's blood pressure to confirm the efficacy of renal denervation at least one month after the procedure. In some cases, treatment failures may be due to regeneration of renal nerves (Esler et al., Lancet 2010, p. 1908), while in others, treatment failures may be due to failure to correctly target and sufficiently complete ablation of the renal nerves. Therefore, methods to precisely detect where renal nerve distribution occurs along the renal arteries, so that ablation targets can be provide to physicians, and to monitor clinically relevant indices (such as blood pressure, heart rate and muscle sympathetic nerve activity) to assess whether efficient ablations are delivered, are urgently needed. As above discussed, renal afferent and efferent nerve system serves as a common pathway for sympathetic hyperactivity, therefore stimulation of renal nerve can cause increases in blood pressure and changes in heart rate. Changes in heart rate can be either increased due to direct stimulation of sympathetic nerves, or decreased blood pressure due to an indirect reflex regulation via baroreflex.
An improved methodology would involve a renal nerve mapping approach by which individual segments of the renal artery are stimulated by a low power electrical current while blood pressure, heart rate and muscle sympathetic nerve activity were measured. If measurable changes in blood pressure, heart rate and muscle sympathetic nerve activity are detected, such as increases in blood pressure or changes in heart rate or decreases in muscle sympathetic nerve activity, there is a reasonable expectation that ablation at that site should be performed so as to destroy nerve fibers in more precise way, and consequently, improve the clinical measures desired. These improved renal nerve mapping and catheterization technologies would seek to minimize unnecessary ablation in the types of denervation procedures described, guide operators to perform renal ablation procedures, and to optimize clinical outcomes of renal nerve ablation for treatment of hypertension, heart failure, renal failure and diabetes.
Anatomical Mapping and Targeting in Renal Nerve Ablation
Anatomically, the nerves carrying fibers running to or from the kidney are derived from the celiac plexus (a/k/a the solar plexus) and its subdivisions, lumbar splanchic nerves, and the intermesenteric plexus (DiBona and Kopp, 1997, p. 79). The celiac plexus consists of the suprarenal ganglion (i.e. the aorticorenal ganglion), the celiac ganglion, and the major splanchnic nerves. The celiac ganglion receives contributions from the thoracic sympathetic trunk (thoracic splanchnic nerves), and the vagus nerves (DiBona and Kopp, 1997, p. 79).
The suprarenal ganglion gives off many branches toward the adrenal gland, some of which course along the adrenal artery to the perivascular neural bundles around the renal artery entering the renal hilus; other branches enter the kidney outside the renal hilar region. The major splanchic nerve en route to the celiac ganglion gives off branches to the kidney at a point beyond the suprarenal ganglion. The celiac ganglion gives off branches to the kidney that run in the perivascular neural bundles around the renal artery entering the renal hilus (DiBona and Kopp, 1997, p. 79).
The lumbar and thoracic splanchic nerves are derived from the thoracic and lumbar paravertebral sympathetic trunk, respectively. They provide renal innervation via branches that go to the celiac ganglion but also via branches that go to the perivascular neural bundles around the renal artery entering the renal hilus (DiBona and Kopp, 1997, p. 79).
The intermesenteric plexus, containing the superior mesenteric ganglion, receives contributions from the lumbar splanchnic nerves and gives off branches that often accompany the ovarian or testicular artery before reaching the kidney (DiBona and Kopp, 1997, p. 79). The renal nerves enter the hilus of the kidney in association with the renal artery and vein (DiBona and Kopp, 1997, p. 81). They are subsequently distributed along the renal arterial vascular segments in the renal cortex and outer medulla, including the interlobar, arcuate, and interlobular arteries and the afferent and efferent glomerular arterioles (DiBona and Kopp, 1997, p. 81).
While the renal nerve architecture is of paramount consideration before ablation can take place, individual renal architecture must be carefully considered before catheterization for denervation can be contemplated. As noted with respect to the Krum et al./Esler et al. studies, eligibility for catheterization was determined in part by an assessment of renal artery anatomy, renal artery stenosis, prior renal stenting or angioplasty, and dual renal arteries. Not only is aberrant or unusual renal architecture an impediment to catheterization in and of itself, but normal variation in renal architecture may prove challenging, especially when an off-label catheter system (i.e. a catheter not specifically designed for renal artery ablation per se) is used. The risks of renal catheterization with sub-optimal catheter systems may include the rupture of renal arteries due to coarse or jagged manipulation of such catheter tips through delicate tissue, rupture of and/or damage to the artery wall or renal artery endothelium due to excessive ablation energy applied, and dissection of the artery. Therefore, catheter systems specially designed for renal architecture and common aberrations in renal architecture are desirable, in order that a large spectrum of the eligible refractory patient population be treated.
Catheter Systems
Certain catheter systems designed for coronary artery systems are similar to those which may be used in renal nerve ablation; in particular, ablative catheter systems designed for coronary artery use which are tailored to remedy tachycardia may be used for renal nerve ablation procedures. As such, these systems typically contain electrodes which are designed to assess the pre-existing electric current in the cardiac tissue through which the catheter electrodes are being passed. In contrast, ideal catheter systems for renal denervation would optimally be engineered with dual functions: to map renal nerve distribution and stimulate renal nerve activity by providing electrical stimulation so that a physician operator may assess in real-time patient physiological changes occurring as a result of said electrical stimulation and renal denervation. However, such catheters have not previously been developed.
Known catheter systems often possess multiple functionalities for cardiac uses. Certain notable catheter systems on the market include the following:
A) Medtronic Achieve™ Electrophysiology Mapping Catheter.
This catheter is normally used for assessment of pulmonary vein isolation when treating paroxysmal atrial fibrillation. It is used in conjunction with Medtronic's Arctic Front cryoablation system. The Achieve™ Mapping Catheter has a distal mapping section with a circular loop which is available in two loop diameters (15 mm and 20 mm). It is deployed through the Arctic Front guidewire lumen, allowing for a single transseptal puncture. The catheter features eight evenly spaced electrodes on a loop, enabling physicians to map electrical conduction between the left atrium and pulmonary veins. Additionally, the catheter allows for assessment of pulmonary vein potential both before and after cryoablation and also helps physicians assess time-to-effect during cryoablation. Its specifications are as follows:
3.3 Fr, 1.1 mm (0.043″) catheter shaft size
165 cm in total length; 146 cm in usable length
Two loop sizes: 15 mm and 20 mm
Two electrode spacings: 4 mm and 6 mm
Eight 1 mm electrodes
Catheter is compatible with minimum ID of 3.8 Fr, 1.3 mm (0.049″)
B) Northwestern University/University of Illinois at Urbana-Champaign All-in-One Cardiac EP Mapping and Ablation Catheter.
This catheter is a combination catheter utilized to perform cardiac electrophysiological mapping and ablations. The balloon catheter includes temperature, pressure, and EKG sensors, and an LED that can ablate cardiac tissue. The catheter is based on a “pop-out” design of interconnects, and the concept of stretchable electronics. In this design, all necessary medical devices are imprinted on a section of a standard endocardial balloon catheter (a thin, flexible tube) where the wall is thinner than the rest; this section is slightly recessed from the rest of the catheter's surface. In this recessed section, the sensitive devices and actuators are protected during the catheter's trip through the body to the heart. Once the catheter reaches the heart, the catheter is inflated, and the thin section expands significantly so that the electronics are exposed and in contact with the heart.
When the catheter is in place, the individual devices can perform their specific tasks as needed. The pressure sensor determines the pressure on the heart; the EKG sensor monitors the heart's condition during the procedure; the LED sheds light for imaging and also provides the energy for ablation therapy to ablate tissue (in this case, typically tachycardia-inducing tissue); and the temperature sensor controls the temperature so as not to damage other healthy tissue. The entire system is designed to operate reliably without any changes in properties as the balloon inflates and deflates.
The system is designed to deliver critical high-quality information, such as temperature, mechanical force, blood flow and electrograms to the surgical team in real time.
C) Medtronic Artic Front®.
The Arctic Front® is an FDA-approved cryoballoon ablation system. The balloon is delivered via the accompanying FlexCath® Steerable Sheath; liquid coolant is pumped in using the CryoConsole control unit. The unit is normally used to treat paroxysmal atrial fibrillation. Its specifications are as follows:
Two balloon diameters: 23 mm and 28 mm
Double balloon safety system
Bi-directional deflection (45 degrees maximum)
Compatible with 12F FlexCath® Steerable Sheath
102 cm working length
D) Diagnostic Products Lasso Circular Mapping Catheter.
The LASSO 2515 Variable Circular Mapping Catheter features a variable loop which adjusts to fit veins sized between 25 and 15 mm.
E) Ardian Symplicity® Catheter System
The current catheter system utilized for renal ablation, comprising both an ablation catheter and radio frequency generator, i.e. the Symplicity® Catheter System, is specially designed by Ardian Inc. (Mountain View, Calif., USA). However, the Symplicity® catheter does not possess mapping functions and ablation is its only function; and secondly, such catheter systems (as well as angioplasty and distal protection devices for angioplasty) were designed for coronary and carotid artery systems—hence, these systems would be used “off-label” for renal nerve ablation and denervation to treat hypertension, heart failure, renal failure and diabetes.
Consequently, with the exception of the Ardian Simplicity® Catheter System, the designs of most of these catheters are not tailored to best fit the anatomy of renal arteries and are for cardiac uses. Therefore, optimized clinical uses of these catheters on renal sympathetic mapping are not possible and clinical effects of these catheters on renal nerve ablation are limited.